1. Field of the Invention
The present invention relates to an active matrix substrate to switch pixel electrodes by switching elements, and a liquid crystal display device utilizing such matrix substrate.
2. Related Background Art
The liquid crystal display device provided with a switching element in each pixel is widely employed in combination with the twisted nematic (TN) liquid crystal, and has been commercialized as the flat panel displays and as the projection television displays. The above-mentioned switching element, represented by a thin film transistor (TFT), a diode element or a metal-insulator-metal (MIM) element, assists the optical switching response of the liquid crystal by maintaining a voltage applied state longer than the actual line selecting period of the TN liquid crystal of a relatively slow response, and provides the liquid crystal lacking the memory property (self-supporting property), such as the TN liquid crystal, with a substantial memory state during a frame period by maintaining the above-mentioned voltage applied state. Also the use of such switching elements in principle avoids crosstalk between the lines or between the pixels, thereby providing satisfactory display characteristics.
In recent years, there has also been developed ferroelectric liquid crystal (FLC) having a faster response in comparison with the TN liquid crystal, and there have been announced a display panel and a light valve utilizing such ferroelectric liquid crystal. The display characteristics may be further improved by the combination of the FLC and the above-mentioned switching elements. The combination of FLC and TFT mentioned above is disclosed, for example, in the U.S. Pat. No. 4,840,462 and in "Ferroelectric Liquid-Crystal Video Display", Proceeding of the SID, Vol. 30, 1989.
In the following there will be explained, as an example, the use of the thin film transistor (TFT) as the switching element. FIG. 4 shows the configuration of a liquid crystal display device provided with a plurality of switching elements corresponding to a plurality of pixel electrodes. In FIG. 4, a glass substrate 1 is provided thereon with a peripheral circuit 8' and TFT's 2 corresponding to pixel electrodes. Between a counter glass substrate 3 opposed to the pixel electrodes and the matrix substrate 1 there is formed an area surrounded by a seal portion 7, and liquid crystal 11 is contained in such area. Color filters may be provided either on the matrix substrate 1 or on the counter glass substrate 3. There are also shown a peripheral circuit portion 8, and an image display portion 9. The counter glass substrate 3 is provided, on the substantially entire area opposed to the matrix substrate 1, with a common transparent electrode 4 composed for example of ITO (indium-tin oxide). On the other hand, the matrix substrate 1 is provided with wirings for signal transfer, but does not basically have an electrode, extended over the entire area, as in the case of the counter substrate 3.
Also the Japanese Patent Application Laid-open No. 7-182996 of the present applicant discloses an active matrix liquid crystal display device utilizing a Si substrate. FIG. 5 is a cross-sectional view showing an example of such liquid crystal display device, wherein shown are a Si substrate 21 constituting the matrix substrate, a field oxide film 22, NSG (non-doped silicate glass) 23 and 29, a counter substrate 3 provided with color filters, a sealant 7, liquid crystal 11, and a transparent insulating film 27. The lower part of the Si substrate 21 is eliminated to obtain a transparent image display area 9. The TFT 2 etc. connected to the pixel electrode are fixed to the field oxide film 22 and the NSG 23. Reference numeral 8 is a peripheral circuit portion.
The present inventor has investigated such conventional structures and have clarified the following drawbacks in the actual handling of the liquid crystal display device.
As explained in the foregoing, since the liquid crystal display device utilizes an insulating substrate such as glass, it is easily affected by electrostatic charge. Electrostatic charge is easily generated by contact with a human body or with a foreign material. The present inventors have tried an experiment to apply charge with an electrostatic probe from the side of the counter substrate and the side of the matrix substrate.
In case of charging from the side of the counter substrate, the display characteristics are scarcely affected even with charging of .+-.10 kV with the electrostatic probe. Such charging amount of .+-.10 kV corresponds to twice of the electrostatic charging amount by the human body (.+-.5 kV) which is ordinarily employed for testing the insulation between the terminals of the IC's.
On the other hand, in case of charging from the side of the matrix substrate, it is found that the display may generate a defect in case the charging amount with the electrostatic probe exceeds -5 kV. Such defect in the display can appear in various forms, such as a bright spot in a black image display, a black spot in a white image display, or a plurality of such defects. This phenomenon is presumably caused by a fact that, because a face of the matrix substrate opposite to the face thereof bearing the pixel electrodes has more chances of contacting foreign matters, an electrostatic charge is induced at first on the rear face of the matrix substrate, thereby correspondingly inducing an electrostatic charge on the front face of the matrix substrate (face bearing the TFT) whereby such electrostatic charge deteriorates the characteristics of the TFT or induces an erroneous function thereof.
The present inventor has estimated that the electrostatic charge from the side of the counter substrate does not affect the characteristics of the TFT because of the shield effect of the common ITO electrode formed on the counter substrate. Based on these facts, the present inventor has reached the present invention.